His Aug. 11 donation to Initiative 594 was made public when it posted on the state's Public Disclosure Commission website Monday afternoon. Campaign manager Zach Silk told The Associated Press that the campaign was grateful for Allen's support.

"He has always been the type of leader who is willing to take on challenging issues and work with people to solve them," Silk wrote in an email.

Other big names have made large donations to the campaign, including former Microsoft CEO Steve Ballmer and his wife, Connie, who have given $580,000, and venture capitalist Nick Hanauer, who has given nearly $400,000 and has pledged an additional $530,000.

Initiative 594 would require background checks for all gun sales and transfers in Washington state, including at gun shows and for private sales. Under the measure, some exemptions would exist, including gifts within a family and antiques. Supporters have now raised nearly $4 million, besides Hanauer's pledge of an additional $530,000.

A rival campaign, Initiative 591, would prevent the state from adopting background-check laws that go beyond the national standard, which requires the checks for sales by licensed dealers but not for purchases from private sellers. That campaign has raised just over $1 million so far.

Alan Gottlieb, chairman for Citizens Committee for the Right to Keep and Bear Arms, who is leading the campaign for I-591, said that the large donation was troubling.

"It obviously concerns me when very wealthy people try to buy an election," he said.